What may be the signs of a bad crank sensor? - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 03-02-2009, 10:24 AM   #1
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Default What may be the signs of a bad crank sensor?

I have a 1994 Bonneville with a non supercharged 3.8. I'll try to explain what happens.

I'll be driving down the road at highway speeds then all of a sudden the car will jerk for a split second because the engine is dying for a split second. I know the engine dies, at least momentarily because then it comes back to life and the transmission has downshifted and the engine revs because it has shifted into a lower gear, as if the engine were shut off. So, I have to let of the gas a little and let the tranny reset back into the proper gear. This doesn't happen all the time. I can go a week maybe, maybe a few days, without it happening then maybe a day or two will go by and it will act up like this.

Plus, when i am driving at lower speeds, coming to a stoplight, or the engine is at idle and this happens, the engine just dies. When it dies, if I'm moving, I can put the car in neutral and it starts back up then i can put it back in drive and continue on.

However, this morning, when i forgot my cell phone (of course) and I was 25 miles away from home it was doing this worse then ever before. It kept dying and I'd have to keep putting it in neutral to restart it but then i wouldn't get far before it would do it again and a few times it died again right away. I was lucky to keep messing with it to get to a gas station and pull in to stop and shut it off.

I let it sit for a couple minutes and started it and it kept dying. I popped the hood and messed around with the wiring a bit because I'm wondering if something is shorting out so i moved wires around and pulled them away from each other, checked my battery cables etc. Started it up and it missed slightly, but i decided to go for it and try to drive home. So I drive through town, stop lights and all, and get back out onto the highway and I made it all the way back home and it only acted up a couple times but not severely. Just like a hiccup, or miss.

I called and talked to a local trusted mechanic, and he said it sounds like a bad crank sensor, or possibly bad harmonic balancer.

Some of you will remember that i just recently replaced a water pump that went out. Could the coolant leaking from the bad water pump have leaked on the sensor, and cause my crank sensor to go bad? Do these problems that I'm having sound like a crank sensor or bad harmonic balancer?
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Old 03-02-2009, 10:39 AM   #2
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Could be a crank sensor. Could be a pcm. Tough to say w/o swapping one for testing.
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Old 03-02-2009, 11:03 AM   #3
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Could be a crank sensor. Could be a pcm. Tough to say w/o swapping one for testing.
Please forgive me because I don't know a lot about 1990 and newer cars with all the electrical and sensors and stuff so I'll ask annoying questions a lot.

What exactly is the pcm? Is it hard to replace?
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Old 03-02-2009, 11:34 AM   #4
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PCM is Powertrain Control Module aka computer
ECM is basically the same, it is what GM called the PCM from about the mid 80'* to the mid 90'*. After that they swapped the name to PCM. Same basic thing though, it runs the engine.

Something to note is that in 94 and 95 GM had the worst luck with the ECM/PCM. Odd unexplained things can usually be remedied after all normal diagnostics are checked by swapping the pcm. This is more of a 94/95 issue than any other year.
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Old 03-02-2009, 11:42 AM   #5
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PCM is Powertrain Control Module aka computer
ECM is basically the same, it is what GM called the PCM from about the mid 80'* to the mid 90'*. After that they swapped the name to PCM. Same basic thing though, it runs the engine.

Something to note is that in 94 and 95 GM had the worst luck with the ECM/PCM. Odd unexplained things can usually be remedied after all normal diagnostics are checked by swapping the pcm. This is more of a 94/95 issue than any other year.
And how expensive of a fix might the pcm be? I'm dreading to hear a price.
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Old 03-02-2009, 11:51 AM   #6
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Local junkyard in Worcestor MA is $20
Advance Auto (last time I checked) ~$80
Overall pretty reasonable.
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Old 03-02-2009, 12:55 PM   #7
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I'm going to take it to the shop and have a diagnostics ran on it. I'll drop it off this afternoon and they will diagnose it tomorrow, I guess. This is the kind of problem that'* over my head.
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Old 03-03-2009, 03:14 AM   #8
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Dude same thing happening with my car. And when I scanned it it came up crank sensor is setting off the light. Check out my thread about the crank sensor and Danthurs (what hasn't he done and photograph I ask) did a whole right up on how to change the senson
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Old 03-05-2009, 11:02 AM   #9
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Dude same thing happening with my car. And when I scanned it it came up crank sensor is setting off the light. Check out my thread about the crank sensor and Danthurs (what hasn't he done and photograph I ask) did a whole right up on how to change the senson
Yeah, I was leaning toward crank sensor also. But the shop can't get it to act up for them to get a code on it. It always works that way. When a machine is acting wrong and you try to show the mechanic/maintanence person what it'* doing, it will run fine. My car will run fine foe a few days then act up. When they first put their analysis machine on it, he said the codes lit up like crazy. I figured that because it has 190,000 miles on it, and I'm sure there are plenty of sensors that need to be cleaned maybe replaced. So he clear them and can't get it to act up to get a reading. Maybe they'll find the problem today.

Mechanic said, it sounds like a crank sensor the way i described what it'* doing, but he said since when it happens I hear the ding, ding, ding and the service engine light comes on he believes it could be some corroded wires maybe going to the pcm.

I hope they can get a reading on it today. I need it fixed. I depend on that car because I'm out of work and going to school full time. School is 45 minutes to an hour drive for me and i have no friends or family that have extra cars or the means to help me back and forth that distance at the times I need it. My wife works so she needs our other car for work and taking the kids to the babysitters. This kind of stuff is such a bummer.

Makes me want to give up and get a car from the late 70'* early 80'* that when something goes wrong, you can directly pinpoint it. No stinking computers, no government enforced sensors BS to deal with. I had several different 1970'* Trans Ams and I was always able to easily do the work to them myself. And they never broke down as often as this 94 Bonneville and our 96 Sable has.
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Old 03-05-2009, 08:21 PM   #10
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Mechanic still couldn't get the car to act up to get a reading on it today. I guess he'* going to try again tomorrow. I think he'* about ready to just let me pay him for the diagnostics and take it home.

If they can't diagnose a problem and just have me take the car, then i guess i can try a couple things.

1. Can anyone tell me where the PCM is located so i can clean and inspect the connection/wires? What do i use to clean the connection?

2. Should I go ahead and replace the crank sensor anyway, even though it looks to be a pain in the rear? (I saw the write up thread by Danthurs here) Speaking of that, when he says to buy a new crank bolt, is it a special bolt or can I go to the local hardware store and get a hardened bolt that will work?

3. How do i clean the MAF sensor? Any other sensors i should clean? or advice you have, I'd love to hear it. You guys have been extremely helpful so far and I certainly appreciate it.

Last edited by rstrunk; 03-05-2009 at 08:24 PM.
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